Conventional methods have failed to create sapphire bodies which exhibit sufficient compressive strength at high temperatures to be useful in many applications. When exposed to elevated temperature, sapphire is generally believed to lose strength at least partially because of "twinning" wherein twin planes slip along adjoining regions. Twinning results in a reorientation of the planes of atoms in one slice of a crystal relative to the alignment in neighboring regions. Twins appear very sharp in cross-polarized light because of the shift in orientation.
Prior attempts to increase the strength of sapphire at elevated temperature have included both ion implantation utilizing particle accelerators and solid solution strengthening methods. Ion implantation techniques have failed, at least in part due to the fact that they only modify the extreme outer portion of a sapphire body due to the shallow range of heavy ion penetration. Solid solution strengthening methods, on the other hand, are based upon introducing substitutional alloys into sapphire during crystal growth by adding small quantities of an inert material during crystal growth. The impurities in the sapphire structure can impede dislocation movements and result in more stable structures, but the materials have still been found to exhibit poor strength or poor optical properties when the sapphire is placed in an elevated temperature environment.
Harris et al, Mechanism of Mechanical Failure of Sapphire at High Temperature, Proc. SPIE, Volume 2286 (1995) discloses that the compressive strength of sapphire (single crystal Al.sub.2 O.sub.3) at 800.degree. C. was reduced by more than 95% of its room temperature value. Harris et al, Mechanical Strength of Sapphire at Elevated Temperature, Proceedings of the 6th DoD Electromagnetic Windows Symposium, Huntsville, AL, Oct. 16-19, 1995 discloses that sapphire specimens had a compressive strength at 800.degree. C. of only from 29 to 45 MPa and similar low compressive strength at 600.degree. C. Others have reported similar relatively low compressive strengths at high temperature when varying the crystal growth method (edge-defines film-fed growth (EFG) vs. heat exchanger method (HEM)), surface finish (as-grown vs. polished), and test atmosphere (air vs. argon).
Dients et al, J. Nucl. Mater., 191-194 (Pt. A), 555-9 (1992) discloses that alumina, aluminum nitride, and silicon carbide exhibit a reduction in bending strength at 400-600.degree. C. after neutron irradiation at 10.sup.24 and 10.sup.26 n/m.sup.2 (10.sup.20 and 10.sup.22 nvt). Unless otherwise specified, "nvt" is used herein refers to a neutron flux in neutrons/cm.sup.2 for neutrons .gtoreq.1 MeV. At 10.sup.26 n/m.sup.2 (10.sup.22 nvt), the mean ultimate bending strength was reported to be 50-60% of the original strength of the material. The loss of strength after irradiation was always accompanied by a large decrease of the Weibull modulus. No considerable difference was found at a fluence of about 5.times.10.sup.24 n/m.sup.2 (5.times.10.sup.20 nvt).
Pells, Radiation Damage Effects in Alumina, Journal of Nuclear Materials 191-194 (1992) 555-559 reports that the a- and c-axes of sapphire increase in length, for 14-meV neutron fluences on the order of 10.sup.20 n/m.sup.2 (10.sup.16 nvt) at 325.degree. K.
Heidinger et al, The Impact of Neutron Irradiation on the Performance of Cryogenically Cooled Windows for Electron Cyclotron Resonance Heating, Fusion Engineering and Design 18 (1991) 337-340 discloses dielectric loss tangent and thermal conductivity calculations for a limited set of data. The calculations are based on data of sapphire irradiated at 3.5.times.10.sup.19 f.n. (fast neutrons)/cm.sup.2, (1.5-50.2).times.10.sup.17 f.n./cm.sup.2 and (0.3-18).times.10.sup.19 f.n./cm.sup.2. Heidinger et al disclose sapphire disks used in gyrotrons and torous windows in devices used for electron resonance heating fusion plasmas.
It has now been unexpectedly discovered that a sapphire body having high strength at elevated temperature can be produced by controlled neutron irradiation processing to introduce point defects within the body.
It is an object of this invention to produce sapphire bodies having increased compressive strength at elevated temperature by irradiating sapphire with a limited amount of fast neutrons.
It is a further object of this invention to develop strengthened sapphire bodies that exhibit desirable transmission characteristics in the midwave region.
These and still further objects will be apparent from the following description of this invention.